Equalization filter and distortion compensating method

Abstract

An equalization filter is provided for solving the problem in which there is a limited range in which compensated for distortion of a transmission signal can be made. Measuring instrument 104 measures a distortion quantity which characterizes distortion of the transmission signal. Comparator 105 a generates a differential signal which indicates the difference between the transmission signal and a compensation signal. Delay device 105 b delays the differential signal based on the distortion quantity measured by measurement instrument 104 and generates the compensation signal.

The invention claimed is: 
     
       1. An equalization filter comprising an input unit for receiving a transmission signal, a generation unit for generating a compensation signal for compensating for distortion of the transmission signal, and a compensation unit for compensating for distortion of the transmission signal based on the compensation signal, further comprising;
 a measuring unit is included for measuring a distortion quantity which characterizes the distortion of the transmission signal wherein: 
 said compensation unit generates a differential signal which indicates the difference between the transmission signal and the compensation signal; and 
 said generation unit delays the differential signal based on the distortion quantity measured by said measuring unit to generate the compensation signal. 
 
     
     
       2. The equalization filter according to  claim 1 , wherein:
 said measuring unit measures the distortion quantity, and then determines whether or not the distortion quantity falls within a particular range; and 
 said generation unit delays the differential signal by a predetermined delay amount when said measuring unit determines that the distortion quantity falls within the particular range. 
 
     
     
       3. The equalization filter according to  claim 2 , wherein:
 said measuring unit adjusts the delay amount of the generation unit in accordance with the distortion quantity when the distortion quantity is determined to fall within the particular range. 
 
     
     
       4. The equalization filter according to  claim 3 , wherein:
 said generation unit includes: 
 an adjustment unit for changing the cycle of a clock signal; and 
 a sample-and-hold type delay unit for fetching the differential signal generated by said compensation unit in synchronism with the clock signal, the cycle of which has been changed by said adjustment unit, when said measuring unit determines that the distortion quantity falls within the particular range, and for generating the fetched differential signal as the compensation signal, and 
 said measuring unit adjusts the cycle of the clock signal, changed by said adjustment unit, in accordance with the measured distortion quantity. 
 
     
     
       5. The equalization filter according to  claim 3 , wherein:
 said generation unit includes a plurality of predetermined delay elements which have delay amounts that are different from one another, and one of which delays the differential signal to generate the compensation signal when said measuring unit determines that the distortion quantity falls within the particular range, and 
 said measuring unit selects a predetermined delay element for generating the compensation signal in accordance with the measured distortion quantity. 
 
     
     
       6. The equalization filter according to  claim 1 , wherein:
 said distortion quantity is represented by a differential group delay or a branch ratio of optical power. 
 
     
     
       7. The equalization filter according to  claim 2 , wherein:
 said distortion quantity is represented by a differential group delay or a branch ratio of optical power, and 
 said particular range is such that said differential group delay is in a range included between N−0.15M [UI] and N+0.15M [UI], and said branch ratio is in a range included between 0.4 and 0.6, and said delay amount of the differential signal is set to a value included between N−0.25 [UI] and N+0.25 [UI], where N is an integer equal to or larger than one, and M is one when N is one and two when N is two or more. 
 
     
     
       8. The equalization filter according to  claim 1 , comprising:
 a rear-stage filter unit for compensating for distortion of the differential signal generated by said compensation unit. 
 
     
     
       9. The equalization filter according to  claim 8 , wherein:
 said rear-stage filter includes at least one feed forward type equalization filter, or at least one feedback type equalization filter. 
 
     
     
       10. The equalization filter according to  claim 1 , comprising:
 a front-stage filter unit for compensating for distortion of the transmission signal received by said input unit, 
 wherein said generation unit generates a differential signal which indicates the difference between the transmission signal which has been compensated for distortion by said front-stage filter unit and the compensation signal. 
 
     
     
       11. The equalization filter according to  claim 10 , wherein:
 said front-stage filter unit includes at least one feed forward type equalization filter, or at least one feedback type equalization filter. 
 
     
     
       12. The equalization filter according to  claim 1 , wherein:
 said compensation unit linearly amplifies the transmission signal and the compensation signal, and generates a differential signal which indicates the difference between the linearly amplified transmission signal and compensation signal. 
 
     
     
       13. The equalization filter according to  claim 12 , wherein:
 said measuring unit adjusts an amplification amount of said compensation unit in accordance with the measured distortion quantity. 
 
     
     
       14. The equalization filter according to  claim 8 , wherein:
 a parameter of said rear-stage filter unit for compensating for distortion of the transmission signal has a fixed value, 
 said compensation unit linearly amplifies the transmission signal and the compensation signal, and generates a differential signal which indicates the difference between the linearly amplified transmission signal and compensation signal, and 
 said measuring unit adjusts an amplification amount of said compensation unit in accordance with the measured distortion quantity. 
 
     
     
       15. The equalization filter according to  claim 10 , wherein:
 a parameter of said front-stage filter unit for compensating for distortion of the transmission signal has a fixed value, 
 said compensation unit linearly amplifies the transmission signal and the compensation signal, and generates a differential signal which indicates the difference between the linearly amplified transmission signal and compensation signal, and 
 said measuring unit adjusts an amplification amount of said compensation unit in accordance with the measured distortion quantity. 
 
     
     
       16. The equalization filter according to  claim 1 , wherein:
 said generation unit assumes the transmission signal and the compensation signal as ternary signals, takes a ternary exclusive logical OR for the transmission signal and the compensation signal, and generates the result of the operation as a differential signal. 
 
     
     
       17. A distortion compensating method performed by an equalization filter comprising an input unit for receiving a transmission signal, a generation unit for generating a compensation signal for compensating for distortion of the transmission signal, and a compensation unit for compensating for distortion of the transmission signal based on the compensation signal, said method comprising:
 measuring a distortion quantity which characterizes the distortion of the transmission signal; 
 generating a differential signal which indicates the difference between the transmission signal and the compensation signal; and 
 delaying the differential signal based on the measured distortion quantity measured to generate the compensation signal. 
 
     
     
       18. The distortion compensating method according to  claim 17 , comprising:
 determining whether or not the distortion quantity falls within a particular range after the distortion quantity is measured, 
 wherein said delaying includes delaying the differential signal by a predetermined delay amount when the distortion quantity is determined to fall within the particular range. 
 
     
     
       19. The distortion compensating method according to  claim 18 , comprising:
 adjusting a delay amount of the differential signal in accordance with the distortion quantity when the distortion quantity is determined to fall within the particular range. 
 
     
     
       20. The distortion compensating method according to  claim 19 , comprising:
 changing the cycle of a clock signal; 
 adjusting the cycle of the clock signal, changed at said cycle changing step, in accordance with the measured distortion quantity; 
 fetching the generated differential signal in synchronization with the clock signal, the cycle of which has been changed, when the distortion quantity is determined to fall within the particular range; and 
 generating the fetched differential signal as the compensation signal. 
 
     
     
       21. The distortion compensating method according to  claim 19 , wherein:
 said generation unit includes a plurality of predetermined delay elements having delay amounts that are different from one another, and 
 said method comprises selecting a predetermined delay element for delaying the differential signal to generate the compensation signal when the distortion quantity is determined to fall within the particular range. 
 
     
     
       22. The distortion compensation method according to  claim 17 , wherein:
 said distortion quantity is represented by a differential group delay and a branch ratio of optical power. 
 
     
     
       23. The distortion compensation method according to  claim 18 , wherein:
 said distortion quantity is represented by a differential group delay and a branch ratio of optical power, and 
 said particular range is such that said differential group delay is in a range included between N−0.15M [UI] and N+0.15M [UI], and said branch ratio is in a range included between 0.4 and 0.6, and said delay amount of the differential signal is set to a value included between N−0.25 [UI] and N+0.25 [UI], where N is an integer equal to or larger than one, and M is one when N is one and two when N is two or more. 
 
     
     
       24. The distortion compensation method according to  claim 17 , comprising:
 compensating for distortion of the generated differential signal. 
 
     
     
       25. The distortion compensating method according to  claim 17 , comprising:
 compensating for distortion of the received transmission signal, 
 wherein said differential includes generating a differential signal which indicates the difference between the transmission signal whose distortion has been compensated for and the compensation signal. 
 
     
     
       26. The distortion compensating method according to  claim 17 , comprising:
 linearly amplifying the transmission signal and the compensation signal; and 
 generating a differential signal which indicates the difference between the linearly amplified transmission signal and compensation signal. 
 
     
     
       27. The distortion compensating method according to  claim 26 , comprising adjusting amplification amounts of the transmission signal and the compensation signal in accordance with the measured distortion quantity. 
     
     
       28. The distortion compensating method according to  claim 24 , wherein:
 a parameter for compensating for distortion of the transmission signal has a fixed value, and 
 said method comprises: 
 linearly amplifying the transmission signal and the compensation signal; and 
 generating a differential signal which indicates the difference between the linearly amplified transmission signal and compensation signal. 
 
     
     
       29. The distortion compensating method according to  claim 25 , wherein:
 a parameter for compensating for distortion of the transmission signal has a fixed value, and 
 said method comprises: 
 linearly amplifying the transmission signal and the compensation signal; and 
 generating a differential signal which indicates the difference between the linearly amplified transmission signal and compensation signal. 
 
     
     
       30. The distortion compensating method according to  claim 17 , wherein:
 said generating assumes the transmission signal and the compensation signal as ternary signals, takes a ternary exclusive logical OR for the transmission signal and the compensation signal, and generates the result of the operation as a differential signal.